Fluoropolymers are known in industry to provide chemical resistance, low moisture absorption and low surface energy surfaces. Fluoropolymers are inert to nearly all chemicals and solvents, even at elevated temperatures and pressures. Fluoropolymers also can function as a barrier as few chemicals are absorbed into or swell fluoropolymers and they possess relatively low gas and moisture permeability. Fluoroplastics are semicrystalline fluoropolymers having a distinct melting point.
Stainless steels are used widely for their corrosion resistance. Although they have extremely good general corrosion resistance, they are nevertheless susceptible to pitting corrosion. Examining a stainless steel surface with a microscope typically shows small pits resulting from intense local corrosion. This local dissolution of an oxide-covered metal in specific aggressive environments is one of the most common and catastrophic causes of failure of metallic structures. Protective coatings, such as a fluoropolymer coating, on stainless steel still may be desirable or even necessary for corrosion resistance in various applications.
While fluoropolymers have been used to coat metallic substrates for non-stick properties (e.g., cookware) and also for corrosion protection (e.g., chemical tanks, exhaust ducts), their non-stick characteristics lead to challenges when bonding fluoropolymers to substrates. Typically the bonding of fluoropolymers to metallic substrates initially involves the use of chemical etching or high pressure grit blasting to give a rough profile to the substrate. A primer is then applied. Known thermally stable binders, such as polyamideimide, polyethersulfone, polyphenylene sulfide, polyetheretherketone, and the like are not known to chemically interact with fluoropolymers, which limits the use of these materials as primers. The primers may be a powder, or more commonly are applied from solvent or via an aqueous solution. The article usually is baked at the necessary temperatures to attain bonding and drive off solvents or liquid carriers. A fluoropolymer topcoat typically is then applied and baked to fuse the fluoropolymer into a protective or decorative coating.